A.c. electrical circuits in buildings are commonly wired such that only one side of the circuit is brought to the location where a wall-switch or thermostat might be mounted. When an electronic controller, such as a timer or the like, is used to replace a common wall-switch or thermostat the controller must obtain all of its operating power requirements from the available a.c. power connections. What this implies is that the current flow through the a.c. power circuit must be adapted to provide any d.c. power needed for the controller's operation.
A prior invention of mine, which now appears as U.S. Pat. No. 4,300,090, recognizes the need for such a "2-wire" line-current operated power supply and develops the voltage drop necessary for developing a d.c. level directly across a thyristor. In this earlier arrangement, no isolation is obtainable between the a.c. line and the d.c. power portions of the circuit. This has not prevented its acceptance for use in a number of commercial products, such as the Model ET-600 time switch manufactured by Paragon Electric Company, Two Rivers, Wisconsin; and the Model EI-15 MH and EJ-341 time switches manufactured by Intermatic, Inc., Spring Grove, Illinois.
The use of a thyristor to develop the necessary voltage drop to operate the d.c. power supply, as done in my earlier patent, deprives the a.c. load of a portion of the available a.c. power. Of course the disadvantageous effects of this drop may be insignificant if the developed d.c. voltage level is relatively low (say, on the order of 9 volts) and the a.c. level is relatively high (say, on the order of 117 volts r.m.s.). However, when the d.c. voltage needs are higher, or the a.c. operating voltage is lower, the voltage drop developed across the thyristor and the power loss from the attendant a.c. load circuit may be unacceptable.
In a building HVAC (heating ventilating air-conditioning) thermostat circuit, 24 volts a.c. is a common operating level. When a smart thermostat using electronic circuits is used, it may be desirable to obtain operating potential for the electronic circuits under any thermostat status condition (i.e., closed, open, or with an anticipator hookup) and with an indefinite circuit current flow which depends upon the kind of zone-valve, relay or other device which the thermostat may control. My invention now provides the development of necessitous operating power levels for the electronic circuitry without causing any significant voltage drop in the overall thermostat circuit hookup, and without the costly need for changing the usual `two wire` hookup typical of most common pre-existing thermostat installations.
My earlier invention, and other known devices producing similar operational advantages, do not provide isolation (electrical separation) between the a.c. power circuit and the d.c. load circuit. In these prior devices, the d.c. circuitry typically may be operating with an a.c. line voltage level above ground imposed upon it. The result is, of course, reduced safety for any persons having access to any of the circuitry portions of a device incorporating the earlier invention. The implied economic disadvantage is that increased (and more bulky and costly) insulation factors are needed, particularly when a product is subject to Underwriter's Laboratory (UL) or other such qualifying agencies requirements. In my instant invention I may obtain full and dependable electrical isolation through the use of a suitable transformer design which alone provides the requisite insulation factors.
In a d.c. power supply based upon my earlier invention, any failure mode of the thyristor that results in non-conduction (e.g., continual open state) of the thyristor will quickly bring about catastrophic failure of the d.c. power supply elements and most likely also any d.c. load apparatus connected with it. This immediate failure of most of the attendant circuit elements occurs because nearly full a.c. line voltage will appear across the thyristor, resulting in excessive rectified d.c. voltage levels. In my instant invention, no active device such as a thyristor is alone used to determine the a.c. levels available for rectification as d.c. power. A small transformer is used, and with suitable design consideration, it can be brought to saturation and become self-limiting before any excessive, potentially damaging, d.c. levels are developed, even if one of the primary current shunt diodes `opens up` due to failure, or if a thyristor is in fact somehow used as the voltage dropping device and for some reason it doesn't `turn-on` properly. The transformer primary winding can furthermore be desinged to `fuse`, thereby opening-up and disconnecting the rest of the apparatus from the line thus removing the danger of fire, or accidental electrocution of an user, which may otherwise be present if the power semiconductor fails.
A fundamental hookup of my invention may include merely a small transformer having a primary winding hooked in series with a load-feeding a.c. power line, and two inverse-parallel connected diodes connected in parallel with the transformer primary winding thereby limiting the voltage drop which may develop across the primary winding impedance due to a.c. current flow. A stabilized secondary source of a.c. power may then be obtained from the transformer secondary winding.
In a most basic expression of my invention as a d.c. power supply merely four circuit elements, employed in novel arrangement, and with the arrangement coupled in series with a source of a.c. electric power and a current-drawing a.c. load, may serve to acheive performance results which, prior to my teaching, would require substantially more complicated and costly apparatus to attain the same level of performance and electrical isolation between the a.c. circuit portion and the d.c. circuit portion. These key elements generally comprise: a bilateral power semiconductor element which might include two: two inverse-parallel connected power diodes; a small transformer; a rectifier diode; and a filter capacitor.
Further refinement of my invention provides for replacement of the inverse-parallel connected power diodes with a thyristor which may be turned-ON through a number of approaches, including obtaining a feed-back trigger signal derived from the main secondary or from a tertiary winding on the transformer; or as controlled by the voltage drop developed across the transformer primary winding impedance which unto itself develops a voltage-drop produced signal which is adequate to turn-ON the thryistor and furthermore where such action may be additionally enhanced through an arrangement which provides a breakdown diode coupled in series with the thyristor gate thus enabling the development of a voltage drop considerably (and controllably) in excess of the mere voltage drop devloped across the ON-state semiconductor junctions alone.